Refactor sprint planning docs and add templates

Updated AGENTS.md with implementation planning conventions and stream index. Refactored SPRINT_110_ingestion_evidence.md, SPRINT_125_mirror.md, and SPRINT_300_documentation_process.md to use a topic-oriented template, clarify dependencies, task boards, and checkpoint structure. Archived previous sprint details and added new templates and status snapshot files to docs/implplan.

AGENTS.md

@@ -224,3 +224,49 @@ You main characteristics:
 - 3. Keep changes deterministic (stable ordering, timestamps, hashes) and align with offline/air-gap expectations.
 - 4. Coordinate doc updates, tests, and cross-guild communication whenever contracts or workflows change.
 - 5. Revert to `TODO` if you pause the task without shipping changes; leave notes in commit/PR descriptions for context.
+
+## 6) Implementation Planning Overview
+
+The implementation plan lives under `docs/implplan`. Follow these conventions whenever you touch sprint artefacts.
+
+- **Numbering & concurrency**
+  - Sprint files use the pattern `SPRINT_<decade><unit>_<topic>.md`.
+  - The **decade** (110s, 120s, …) identifies a delivery stream. Streams can execute in parallel.
+  - Numbers inside the same decade (e.g. `110`–`119`) **must not depend on each other**; they are intentionally concurrency-safe once upstream decades have landed.
+  - Multi-phase work keeps the decade and adds suffixes (`_i`, `_ii`, …) while preserving the no-intra-decade-dependency rule.
+- **Required sections per sprint file**
+  1. `## Topic & Scope`
+  2. `## Dependencies & Concurrency`
+  3. `## Documentation Prerequisites`
+  4. `## Task Board` (single table with `Task ID`, `Status`, `Owner(s)`, `Dependencies`, `Notes`)
+  5. `## Execution Log`
+  6. `## Decisions & Risks`
+  7. `## Next Checkpoints`
+     - Optional deep dives go into `docs/implplan/archived/` or linked supplements to keep sprint files concise.
+- **Updating sprint status**
+  - Flip the relevant row to `DOING` as soon as you start and keep the parent stream tracker in sync (e.g. Sprint 300 for Docs).
+  - Revert to `TODO` with a log entry if you pause mid-work.
+  - Mark `DONE (YYYY-MM-DD)` and capture validation evidence in the execution log when closing work.
+- **Creating new sprints**
+  1. Copy `docs/implplan/templates/sprint-template.md` to `docs/implplan/SPRINT_<id>_<topic>.md`.
+  2. Update headings, scope, dependencies, and seed the initial task board.
+  3. Register the sprint in the stream index below and any parent tracker.
+- **Stream index (update when scopes change)**
+
+  | Decade | Stream | Notes |
+  | --- | --- | --- |
+  | 110s | Ingestion & Evidence | Advisory AI, Concelier, Excititor, Mirror. |
+  | 120s | Policy & Reasoning | Findings ledger, policy evaluation, provenance. |
+  | 130s | Scanner Surface | Scanner surface hardening and deterministic compose. |
+  | 140s | Signals & Graph | Runtime signals, graph enrichment, Zastava. |
+  | 150s | Orchestration & Scheduling | Scheduler, orchestrator, automation packs. |
+  | 160s | Evidence & Export | Export Center, Evidence Locker, timeline indexer. |
+  | 170s | Notifications & Telemetry | Notify stack and telemetry improvements. |
+  | 180s | Replay Foundations | Deterministic replay primitives and evidence locker integration. |
+  | 200s | Experience SDKs & CLI | CLI, SDKs, dev portal, experience alignment. |
+  | 210s | UI & Web | Console SPA and web surfaces. |
+  | 300s | Documentation Process | Docs process ladder and per-module dossiers. |
+  | 400s | Reachability & Runtime Evidence | Runtime callgraph union and reachability evidence chain. |
+  | 500s | Ops & Air-gap | Offline modes, deployment, samples, sovereign crypto. |
+
+Keep this index synchronized any time new sprints are added or re-scoped.